In a fuel cell having a solid polymer membrane as a PEM (proton exchange membrane) made of an electrolytic substance, this electrolytic membrane is disposed between a “region supplied with a fuel to be dissociated to promote an anodic action” (herein called “anode”) and a “region supplied with air as an oxydant for promoting a cathodic reaction” (herein called “cathode”). At the anode, where hydrogen is supplied as the fuel, this hydrogen (H2) is dissociated into hydrogen ions (H+) and electrons (e−), such that H2→2H++2e−. The hydrogen ions are transported through the electrolytic membrane, from the anodic side to the cathodic side. The electrons are conducted from the anode to the cathode through external circuitry connected therebetween, thereby supplying electric power to a load in the circuitry.
At the cathode, oxygen (O2) in the air supplied thereto reacts on hydrogen ions (H+) transported thereto and electrons (e−) conducted thereto, producing water (H2O) to be ejected outside the fuel cell, such that ½O2+2H++2e=→H2O. Therefore, in the fuel cell, as a whole, water (H2O) is produced from hydrogen (H2) supplied as a fuel to the anode and oxygen (O2) in the air supplied to the cathode, such that H2+½O2→H2O. The hydrogen and air are sometimes each respectively called “reaction gas”.
The electrolytic membrane needs to be saturated with water contained therein to serve as an ion-transportable electrolyte, and to function for isolation between hydrogen as a reaction gas at the anode and oxygen as a reaction gas at the cathode. If the water content is insufficient, the electrolytic membrane has a decreased ion mobility and/or causes an unfavorable mixing between reaction gases, resulting in a failure for the fuel cell to generate sufficient electric power.
When dissociated hydrogen ions are transported through the electrolytic membrane from the anodic side to the cathodic side, water is moved together, so that the electrolytic membrane inherently has a drying tendency at the anodic side. The water content of the electrolytic membrane is supplemented by moisture contained in reaction gas, which is supplied through gas inlet ports near the electrolytic membrane. If the moisture content of reaction gas is insufficient, the electrolytic membrane tends to be dried in regions vicinal to the gas inlet ports.
It therefore is necessary for a solid polymer membrane, as an electrolytic membrane in a fuel cell, to be moisturized by an externally supplied “moisture or hydration water” (herein collectively called “moisture”). Techniques for such “moisturization or hydration” (herein collectively called “moisturization”) are disclosed in U.S. Pat. No. 5,284,718 in which fuel and air are let through semi-permeable layers in a humidification section of a fuel cell, where they are humidified, before entering an active section for their reaction, and in Japanese Patent Application Laid-Open Publication No. 11-162490 in which fuel and air are let through supply pipes, where they are humidified by externally supplied water vapor.